Drive assembly for a four wheel drive vehicle, having a disconnectable viscous coupling

ABSTRACT

A drive assembly for a four wheel drive vehicle, which drive assembly, during forward driving, automatically assumes a freewheeling position if the speed of the dependent wheel axle is higher than that of the permanently driven axle, with the four wheel drive function otherwise being maintained with a reduced driving torque, even during reversing, the drive assembly providing the freewheeling unit with a second viscous coupling which is connected in parallel and whose torque transmitting capacity is smaller than, or equal to, that of the first viscous coupling.

This is a continuation of U.S. patent application Ser. No. 602,188,filed Oct. 22, 1990 now U.S. Pat. No. 5,174,408 issued Oct. 29, 1992.

BACKGROUND OF THE INVENTION

The invention relates to drive assembly for a four wheel drive vehiclein the case of which the wheels of the front axle are driven permanentlyand the wheels of the rear axle are driven by a viscous couplingarranged in the longitudinal driveline. A freewheeling unit is connectedin series with the viscous coupling and, during forward driving, permitsthe rear wheels to rotate faster than the front wheels.

From U.S. Pat. No. 3,760,922 it is known to provide drive assemblieswith viscous couplings having at least two sets of plates with one seteach being associated with a coupling hub and a coupling housing. Theremaining space between the alternately arranged set of plates ispartially filled with a viscous fluid, e.g. silicone oil.

In the case of drive assemblies for four wheel drive vehicles providedwith viscous couplings, it is temporarily necessary to disconnect therear axle from the engine drive, for example when operating the vehiclebrake in order to maintain the stability of the vehicle.

From DE 33 17 247 A1 it is known, for example, to provide the rear axlewith a torque derived from the front axle differential of a vehiclepermanently driven via the front wheel axle. A viscous coupling isincorporated into the longitudinal driveline of such a vehicle, so thatif a speed differential between the two wheel axles occurs, anincreasing driving torque is transmitted to the rear wheels. A speeddifferential between the two axles occurs of necessity for example ifthe permanently driven front wheels are subject to a higher degree ofslip. As a result, an additional torque builds up for the rear axlebecause the speed differential between the two axles causes a relativemovement of the sets of plates relative to each other, with the viscousfluid in the coupling housing being sheared. Under normal drivingconditions this effect is desirable.

However, the disadvantage of this design of a viscous coupling in thedriveline is that independently of how the speed differential occurs,the viscous coupling changes into the torque transmitting condition assoon as a speed differential occurs. If the vehicle is braked, theshearing effect of the viscous fluid also causes a braking moment to betransmitted to the rear axle via the viscous coupling, so that anincreased slip occurs at the rear axle, with the vehicle losing itslateral stability.

Even with vehicles with an anti-locking device, this disadvantage isparticularly noticeable.

To avoid such a critical driving condition, it is known to design theviscous coupling in such a way that, if necessary, it may bedisconnected via a switching assembly. If the rear axle is subject to ahigher speed than the front axle, the switching assembly acts like afreewheeling unit and prevents the occurrence of such critical drivingconditions when the front axle is over-braked or the vehicle is subjectto a load change.

For this reason, the patent specification already mentioned proposes apermanently effective freewheeling unit with a freewheeling torque beingtransmitted from the front axle to the rear axle during forward driving.During reversing under four wheel drive conditions, the freewheelingunit is blocked manually or automatically, for example by engaging thereverse gear. The disadvantage of this design refers to the mechanicalblocking of the freewheeling unit between the front and rear axle duringreversing. Furthermore, it is necessary to provide a switching devicefor operating the freewheeling lock, which renders the production ofsuch a drive assembly more expensive.

From DE 37 08 193 A1 it is known to provide a permanently ineffectivefreewheeling device with a freewheeling lock which, when operating thevehicle brake, automatically unlocks the freewheeling lock and switchesthe freewheeling unit into an operative condition. However, thedisadvantage of this design are that the freewheeling device can only beswitched via the vehicle brake and that the rear axle cannot bedisconnected when changing to a traction mode of operation.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a driveassembly which during forward driving automatically assumes afreewheeling position if the speed of the dependent wheel axle is higherthan that of the permanently driven wheel axle and which, otherwise,retains the function of a four wheel drive with a reduced driving torqueeven during reversing.

Pursuant to this object, and others which will become apparenthereafter, one aspect of the present invention resides in a secondviscous coupling connected in parallel with the freewheeling unit, whosetorque transmitting capacity is smaller than, or equal to, that of thefirst viscous coupling.

During forward driving the braking torque acts on the rear axle via thesecond viscous coupling and during reversing torque is transmitted viathe two viscous couplings connected in series.

By providing a smaller torque transmitting capacity, blocking of therear wheels via the front wheels is prevented.

According to a further embodiment of the invention, the second viscouscoupling has a hump effect. Because the second viscous coupling has ahump effect, a further reduction in torque is avoided.

According to yet another embodiment of the invention, the output end ofthe freewheeling unit is non-rotatingly connected to the output end ofthe second viscous coupling. In the case of this design, driving iseffected via the transmitting connection between the first viscouscoupling and the freewheeling unit during forward driving, with thesecond viscous coupling, initially, not having any function because thetwo sets of plates have the same speed.

When the axle which is not permanently driving exhibits a higher speed,the freewheeling unit is activated automatically, thereby permittingovertaking. In this case, the existing transmitting connection betweenthe two viscous couplings connected in series has a braking effect onthe dependent wheel axle. If the direction of rotation is reversed,torque is transmitted via the two viscous couplings connected in series.

In a further embodiment of the invention, the two viscous couplings,together with the freewheeling unit, are accommodated in a housing.

By associating the two sets of plates with the freewheeling unit it ispossible to achieve a particularly advantageous drive assembly designrequiring no more than the smallest possible space in the vehicle.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a four wheel drive vehicle having a drive assembly for therear axle pursuant to the present invention;

FIG. 2 shows a diagrammatic view of a drive assembly for a propellershaft for transmitting torque to the rear axle;

FIG. 3 is a drive assembly according to FIG. 2 with a markedtransmitting function in the forward direction;

FIG. 4 is a drive assembly according to FIG. 2 with a markedtransmitting function during braking and in the case of a load change;

FIG. 5 is a drive assembly according to FIG. 2 with a markedtransmitting function during reversing; and

FIG. 6 is a drive assembly with a compact design.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a four wheel drive motor vehicle 1 whose engine 2, via amanual gearbox 3 and a front axle differential 4 and front halfshafts 5is connected to driven front wheels 6. In addition, the drive for therear axle is branched off the front axle differential 4 via a propellershaft 7. The propeller shaft 7 includes a drive assembly 8 which, via arear differential drive 9 and halfshafts 10, is connected to the rearwheels 11.

FIGS. 2 through 5 show the operating principle of the drive assembly 8having a first viscous coupling 12 associated with the input end and asecond viscous coupling 13 associated with the output end of thepropeller shaft 7. Both viscous couplings 12, 13 comprise inner plates14, 15 and outer plates 16, 17 and are connected to each other via afreewheeling unit 18. The freewheeling unit 18 consists of an inner ring19 and an outer ring 20, with the outer plates 16 of the first viscouscoupling 12 being connected directly to the propeller shaft 7 and withthe inner plates 14 being connected to the outer ring 20 of thefreewheeling unit 18 and, via the freewheeling unit 18, to the outerplates 17 of the second viscous coupling 13. The inner ring 19 of thefreewheeling unit 18 is connected directly to the inner plates 15 of thesecond viscous coupling 13.

A line 21 in FIG. 3 indicates the function of the drive assembly 8during forward driving. The outer plates 16 of the first viscouscoupling 12 which are firmly connected to the propeller shaft 7, as aresult of the shear effect of the viscous fluid, move the inner plates14 and drive the outer ring 20 of the freewheeling unit 18. Via a lock25 not explained in greater detail, the outer ring 20 transmits thetorque onto the inner ring 19, with the lock 25 in an engaged position,which, in turn, is directly connected to the output end of the propellershaft 7. Because the vehicle 1 is accelerated in the forward direction,a slip occurs at the front wheels 6, and the speed differential betweenthe front wheels 6 and the rear wheels 7 causes a torque in the firstviscous coupling 12 as a result of the shear effect of the viscousfluid. Because the outer ring 20 of the freewheeling unit 18 isnon-rotatingly connected to the inner ring 19, torque is transmitteddirectly to the rear wheels 11. The second viscous coupling 13, in thiscase, has no function because the inner and outer plates 15, 17 have thesame speed.

Under these normal driving conditions, and taking into account thecharacteristics of the first viscous coupling, the vehicle operatesunder four wheel drive conditions, moving at an accelerated or uniformspeed.

The line 21 in FIG. 4 indicates the function of the drive assembly 8 inthe case of a load change and when braking during forward driving. Ifthe speed of the front wheels 6 is lower than that of the rear wheels11, the input end is driven by the propeller shaft 7 only via the innerplates 15 and the outer plates 17 of the second viscous coupling 13. Thefreewheeling unit 18, in this case, is disengaged and there is noconnection between the outer ring 20 and the inner ring 19 since thelock is disengaged. Under these delayed operating conditions due to thefuel supply being throttled or with the vehicle brake being operated, anadditional braking effect is exerted on the rear axle 11 by theconnected engine 2 with the manual gearbox 3 via the second viscouscoupling 13 so that four wheel drive conditions remain effective.Because of the smaller capacity of the second viscous coupling 13,locking of the rear wheels is impossible and as a result of the lateralguidance and traction forces of the rear wheels 11 the stability of thevehicle remains completely unchanged.

The line 21 in FIG. 5 indicates the function of the drive assembly 8during reversing. In this case, the direction of rotation of thepropeller shaft 7 is reversed and the freewheeling unit is not blocked,with the speed of the front wheels 6 being higher than that of the rearwheels 11. The torque is transmitted from the first viscous coupling 12via the outer ring 20 of the freewheeling unit 18 to the second viscouscoupling 13 and thus to the output end of the propeller shaft 7. Becauseof the smaller capacity of the viscous coupling 13, the entire torque isnot available. However, even during reversing the four wheel drivefunction is maintained.

FIG. 6 shows a compact design of the drive assembly 8' which, via aconnection flange 22 and a short driveshaft 23, may be bolted to thegearbox 3 and which, at the output end, comprises a connection with thepropeller shaft 7. In this case, the connecting flange 22, via thedriveshaft 23, is connected to the inner plates 14 of the first viscouscoupling 12', with the outer plates 16, via a housing 24, being directlyconnected to the outer ring 20' of the freewheeling unit 18' and theouter plate 17 of the second viscous coupling 13'. The inner ring 19' ofthe freewheeling unit 18' is connected to the inner plates 15 of thesecond viscous coupling 13' and to the propeller shaft 7. Thisparticularly compact design means that the drive assembly only requiresa smallest possible space in the vehicle.

While the invention has been illustrated and described as embodied in adrive assembly for a four wheel drive vehicle, having a disconnectableviscous coupling, it is not intended to be limited to the details shown,since various modifications and structural changes may be made withoutdeparting in any way from the spirit of the present invention,

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by letters patent isset forth in the appended claims:
 1. A drive assembly for a four wheelvehicle, comprising:a front axle with wheels which are drivenpermanently; a rear axle with wheels which are driven by a viscouscoupling arranged in a longitudinal driveline; a freewheeling unitconnected along the driveline in series with a viscous coupling so thatduring forward driving the freewheeling unit allows the rear wheels torotate faster than the front wheels; a second viscous coupling connectedalong the driveline in parallel with the freewheeling unit, said secondviscous coupling having a torque transmitting capacity no greater thanthat of the first viscous coupling; and a housing, the first and secondviscous couplings, together with the freewheeling unit, beingaccommodated in the housing.